There has been limited use of fuel injectors for automobile engines for many years. However, it has only been recently with the emphasis on the need to combine good engine performance with reduced emissions that the use of fuel injectors has become more widespread in the automotive industry. Injectors permit the flow of fuel through the engine combustion chambers to be much more precisely metered than is possible with conventional carbureted engines, so that the combustion can be better controlled to provide adequate engine power while reducing the amount of unburned fuel exhausted from the engine.
In order to function properly, fuel injectors are manufactured with precisely dimensioned nozzles and control valves. When the injectors are new and clean, they meter the proper amount of fuel into the combustion chamber during each injection cycle. As the engine service life progresses, however, the injector nozzles and valves gradually become coated with carbon from combustion, oil from cylinder blow-by, resins and lacquers which separate from the fuel, and similar deposits. All of these serve to clog and narrow fuel passages, thus restricting the amount of fuel which is introduced into the combustion chamber in each injection cycle. The reduced amount of fuel, of course, adversely affects engine performance, and the less efficient engine performance in turn increases engine emissions. Particularly poor engine performance occurs when the fuel injectors in an engine accumulate deposits at different rates, so that the engine's cylinders are receiving different quantities of fuel with each cycle. This is most noticeable by the tendency of the engine to run irregularly with considerable vibration, commonly known as "running rough."
A certain amount of deposit build-up is normal in engine service, and both fuel injectors and engines are designed to accommodate such routine deposits. However, maintenance of proper performance of an engine requires that the condition of the fuel injectors be monitored periodically so that excessive deterioration of fuel flow characteristics can be discovered at an early stage when chemical cleaning techniques can be effectively employed. Monitoring is also important where rough running of the engine indicates that an individual fuel injector may have significantly higher deposits than the others in that engine.
Fuel injector performance monitoring has, however, been quite difficult in the past. The recommended methods have been of two types. In the first method, all injectors are removed from the intake manifold but remain attached to the fuel lines. Then with the engine cranked for a predetermined time interval, the ejected fuel is externally collected in graduated containers, one for each injector. This procedure requires extensive labor on the part of the mechanic. In addition, spraying fuel into open containers in an engine compartment is quite hazardous. The second type of test method involves measuring the pressure drop in the fuel system with a pressure gauge. Initially the fuel system is brought up to operating pressure by turning on the ignition, thus automatically actuating the fuel pump. Next the ignition is turned off, but the fuel pressure persists owing to a check valve in the pump. Whenever a single injector is actuated by an external circuit, the pressure abruptly falls, because of liquid flow out of the injector. The magnitude of the pressure drop depends upon the amount of liquid ejected. However, the pressure drop is also dependent upon the compressibility of the test system, which is critically determined by the amount of air trapped in the pressure gauge line. For example, the less air trapped the greater will be the pressure drop. In this type of test system there is no means for precisely controlling the trapped air or bubbles in the gauge line; hence the readings are subject to considerable inaccuracy. Furthermore, there is no fixed relationship between the ejected fluid quantity and the pressure drop for a test system; given a specific pressure drop, it is not possible to calculate the quantity of fluid ejected.
There have also been numerous ways of measuring liquid volume changes or flow rates in the past. Liquid storage tanks with sight glasses to show liquid level drops are common. The accuracy of the measurement is poor, however, since the sight glass level change is equal to the bulk liquid level change, so small changes in volume are difficult to detect. Flow metering devices, such as rotameters, are also widely used, but these are bulky and not accurate for the pulsed liquid flow applicable to fuel injector operation. In addition, liquid flow rate varies during a run (from zero when the valve is opened to maximum and back to zero as the valve is closed), so for short time intervals flow rate cannot be accurately converted to liquid quantity measurements.
It would therefore be of significant value to have a testing device and method which could be easily used by a mechanic or a capable car owner to check the fuel flow condition of individual fuel injectors in an engine, without having to remove the fuel injectors from the engine. Such a device should also permit the condition measurement to be sufficiently accurate to provide the user with a precise comparison between injectors. In addition, the device should be simple and durable so that it could be readily used in the environment of a garage or repair shop. It should also be useful for other types of fuel injected engines.